Synthesis of Alstoscholarisines A-E, Monoterpene Indole Alkaloids with Modulating Effects on Neural Stem Cells

Direct Conversion of 3-(2-Nitroethyl)-1H-Indoles into 2-(1H-Indol-2-yl)Acetonitriles

The recently discovered [4+1]-spirocyclization of nitroalkenes to indoles provided a convenient new approach to 2-(1H-indol-2-yl)acetonitriles. However, this reaction was complicated by the formation of inert 3-(2-nitroethyl)-1H-indole byproducts. Herein, we offer a workaround this problem that allows for effective transformation of the unwanted byproducts into acetonitrile target molecules.

Enantioselective Total Synthesis of (+)-Alstilobanine C, (+)-Undulifoline, and (-)-Alpneumine H

We report herein the enantioselective total synthesis of three monoterpene indole alkaloids, namely, (+)-alstilobanine C, (+)-undulifoline, and (-)-alpneumine H. The key features of our synthesis include: a) introduction of chirality via enantioselective deprotonation of a prochiral 4-substituted cyclohexanone; b) use of methoxymethyl (MOM) ether as both a hydroxyl protective group and a latent oxonium species for the formation of bridged oxepane and c) domino double reductive cyclization to build both the indole and the piperidine ring at the end of the synthesis. The synthesis confirmed the absolute configuration of these natural products assigned based on the biogenetic hypothesis.

An Overview of Saturated Cyclic Ethers: Biological Profiles and Synthetic Strategies

Saturated oxygen heterocycles are widely found in a broad array of natural products and other biologically active molecules. In medicinal chemistry, small and medium rings are also important synthetic intermediates since they can undergo ring-opening and -expansion reactions. These applications have driven numerous studies on the synthesis of oxygen-containing heterocycles and considerable effort has been devoted toward the development of methods for the construction of saturated oxygen heterocycles. This paper provides an overview of the biological roles and synthetic strategies of saturated cyclic ethers, covering some of the most studied and newly discovered related natural products in recent years. This paper also reports several promising and newly developed synthetic methods, emphasizing 3-7 membered rings.

Alstonlarsines A-D, Four Rearranged Indole Alkaloids from Alstonia scholaris

Four indole alkaloids, alstonlarsines A-D (1-4), were isolated from Alstonia scholaris and structurally characterized. Compound 1 possesses a new carbon skeleton with a cage-shaped 9-azatricyclo[4.3.1.0^3,8]decane motif, and compounds 2-4 feature a rare carbon skeleton that was found in nature for the first time. Plausible biosynthetic routes for 1-4 are proposed. Compound 1 showed DRAK2 inhibitory activity with an IC₅₀ value of 11.65 ± 0.63 μΜ.

Alstoscholactine and Alstolaxepine, Monoterpenoid Indole Alkaloids with γ-Lactone-Bridged Cycloheptane and Oxepane Moieties from Alstonia scholaris

Two new monoterpenoid indole alkaloids, alstoscholactine (1) and alstolaxepine (2), were isolated from Alstonia scholaris. Compound 1 represents a rearranged stemmadenine alkaloid with an unprecedented C-6-C-19 connectivity, whereas compound 2 represents a 6,7- seco-angustilobine B-type alkaloid incorporating a rare γ-lactone-bridged oxepane ring system. Their structures and absolute configurations were determined by spectroscopic analyses. Compound 1 was successfully semisynthesized from 19 E-vallesamine. Compound 2 induced marked vasorelaxation in rat isolated aortic rings precontracted with phenylephrine.

The Alstoscholarisine Alkaloids: Isolation, Structure Determination, Biogenesis, Biological Evaluation, and Synthesis

The alstoscholarisines are a small family of biologically and structurally interesting polycyclic monoterpenoid indole alkaloids isolated from the leaf extracts of Alstonia scholaris. The alkaloids can be divided into three different subtypes based upon their structures and putative biogenesis: (1) (-)-alstoscholarisines A-E, (2) (+)-alstoscholarisine G, and (3) (+)-alstoscholarisines H-J. This review discusses the isolation, structure determination, biological activity, and proposed biosynthesis of these metabolites. In addition, synthetic studies on the alkaloids are described including total syntheses of racemic alstoscholarisines A-E, a total synthesis of (-)-alstoscholarisine A, and a synthesis of racemic alstoscholarisine H.

Enantioselective and Divergent Syntheses of Alstoscholarisines A, E and Their Enantiomers

Concise, enantioselective, and divergent syntheses of alstoscholarisines A and E are presented in 8 and 9 steps, respectively; alstoscholarisine E has never been accessed before. A boron-mediated aldol reaction and Rh-catalyzed cycloisomerization were exploited to access stereoisomers 8 and 9 as key intermediates. The challenging sterically congested alstoscholarisine core was furnished by a reductive transannular cyclization in the final steps. This strategy was also used for the syntheses of enantiomers of alstoscholarisines A and E.